This application claims the priority of German application 197 38 621.0, filed in Germany on Sep. 4, 1997.
The invention takes its departure from a universal joint of the type described in German Patent Document DE 43 04 274C1.
Universal joints of this type, when used in vehicles, are not only subjected to very high mechanical loads as a result of torque peaks and by movements of the joint that are excessive, but there is also a thermal stress on the universal joint caused by heat conduction and heat radiation from parts in the vicinity, some of which are very hot. As a result of the heating of the parts, the loading capacity of the materials used is reduced in the case of plastic parts so that the thermal stress on the universal joint is relevant to its mechanical loading capacity.
Many known universal joints, like those provided in particular for lightweight designs of propeller shafts, have at the ends of the two shafts joined by the joint, flanges with three driving tabs projecting axially in the direction of the universal joint, with the total of six driving tabs, each located opposite a gap, being arranged uniformly around the circumference of a common partial circle. The driving tabs are connected nonrotatably with one another by a joint ring that is flexible, hexagonal, and has sockets at the corner points. The joint ring is axially elastically deformable between the sockets so that the two flanges connected by the joint ring can perform (limited) angular movements in any desired direction, even during rotation. As a result of this angular movement, the joint ring becomes axially corrugated, so that within the scope of elasticity in accordance with Hooke's law, bending stresses result that are superimposed on the operating stresses produced by the torque. As a result of this superimposition of stresses, the limit of the strength of the material can be reached. Accordingly, for reasons of long-term strength, universal joints can be allowed only limited bending angles.
The number of three drive tabs per flange, in other words a total of six driving tabs in the universal joint, is not absolutely necessary. It is also possible to have only two tabs per flange, in other words a total of four driving tabs, but because of the distribution of the circumferential load on this small number of driving elements, the latter are subjected to greater stress than when the number of driving tabs is greater. In addition, four or more tabs per flange, in other words a total of eight or more driving tabs, are also possible in the universal joint, and as the number of driving tabs increases, the required diameter of the joint increases and/or the tolerable bending angle becomes smaller. For this reason, the number given above of three to six driving tabs represents the most frequent compromise between the loads on individual parts on the one hand and the size of the joint on the other.
German Patent Document DE 41 40 311 A1 shows a hexagonal joint ring made of composite fiber material in the form of an annular disk in which axial sockets are formed in the reinforced corner areas. The rectilinear areas between two adjacent corners are formed by a relatively thin rib extending in the plane of the annular disk and composed of several combined layers of cured bonded fiber material. This rib is relatively wide in the radial direction. When the universal joint bends, these ribs are not only bent but also twisted. Although the thin ribs can bend elastically at relatively low bending stresses, because of the shape of the ribs, which differs sharply from rotational symmetry, high stresses develop when they are twisted, particularly at their inner and outer edges.
German Patent Document DE 43 04 274 C1 mentioned at the outset teaches a hexagonal joint ring in which the sockets located in the corner areas are connected with one another pairwise by endless oval driving loops wrapped externally around two adjacent sockets. Variable-width, centrally located driving loops and a pair of narrow driving loops located at the edge are provided in the joint ring. The wide driving loop is approximately three times as wide as its radial wall thickness; the two narrow driving loops together are approximately the same width. The driving loops can pivot radially on the sockets so that the joint ring can be changed in shape with articulation as an individual part. To hold together all the parts of the joint ring and to prevent dirt from entering, the parts of the joint ring are embedded in a rubber-elastic material of low hardness and completely enclosed. Because the wall thickness is relatively small in relation to the axially measured width of the driving loops, they offer good conditions for accepting the circumferential pull with low excessive stresses at the curves of the driving loops. When this universal joint is bent, the driving loops are tilted or inclined in the axial direction and are also twisted, so that considerable excess stresses develop at various points in the driving loops. During tilting, there is a unilateral edge contact between the loop curve and the socket and accordingly there is a concentration of stresses at that point. When the relatively wide driving loops are twisted, similarly to the ribs of the previously described joint disk according to German Patent Document DE 41 40 331 A1, excessive stresses develop at the edges, though at a lower level than in the other case. This type of joint ring therefore does not offer any good conditions for good mobility of the universal joint when being simultaneously subjected to a high torque.
A goal of the invention is to improve the universal joint that serves as the basis according to the species in such fashion that relatively high bending angles can be permitted readily during rotation under high mechanical and thermal loads.
Taking its departure from the basic universal joints of the type referred to above, this goal is achieved according to certain preferred embodiments of the invention by providing a universal joint for joining two shafts such that they are axially and angularly movable within certain limits with respect to one another, comprising: shaft flanges located at facing ends of the two shafts, each of said shaft flanges having a plurality of axially projecting driving tabs with respective driving tabs of one shaft flange being disposed in use intermediate respective driving tabs of the other shaft flange along a common circle surrounding an axis through said universal joint, and a joint ring assembly connecting the respective driving tabs with one another, said joint ring assembly including: respective driving tab accommodating sockets disposed at respective corner points of a polygonal configuration, and oval driving loops wrapped around respective pairs of adjacent sockets, said driving loops having a rectangular cross-sectional shape with respective longer cross-sectional sides extending in a radial plane of the universal joint and respective shorter cross-sectional sides extending axially of the universal joint, the longer cross-sectional sides of the supporting cross-sections of the driving loops being at least four times longer than the shorter cross-sectional sides, said driving loops forming legs extending between adjacent sockets surrounded by the respective driving loops, which legs are axially yieldable and elastically twistable, wherein the sockets are provided with position-stablizing circumferential grooves at least in areas wrapped by the driving loops, said grooves matching the cross-sectional shapes of the driving loops so as to receive the driving loops in a position-stablizing manner while permitting the loops to bend and twist elastically like leaf spring tongues, wherein lateral flanks of the circumferential grooves of the socket expand in a circumferential area not wrapped by the driving loops, and wherein the sockets are rotationally asymmetrical and are secured against twisting within the joint ring.
According to preferred embodiments of the invention, the driving loops secured in a stable position on the sockets can bend and twist elastically like leaf spring tongues, so that because of the axial travel and the pivoting movements that the driving tabs perform with respect to one another during operation, the driving loops accordingly are subjected only to a readily tolerable stress. Because of this, relatively large bending angles and/or relatively high ambient temperatures can be permitted.